This may be a really stupid question, but here goes! I read on here a lot about the advantages about buying the same brand of machine so you can share feet. If you end up with both 5.5 and 9 mm. machines, is there a problem using the thinner feet on the wider feed dogs, and vice-versa?

i have used pfaff feet on a 9mm, 6mm, and 7mm machine without any issue. in the accessory book, it lists the machines that are compatible with the different feet. i'm not sure if the coded feet on the berninas work the same way though.

my disclaimer would be to manually turn the handwheel to make sure that the needle isn't going to strike the foot just to be safe! :)

------it doesn't matter what type of sewing you do. you are sewing, and sewing is good.

Regarding Berninas--
If you put a 5.5mm foot on a 9mm machine, only the center-most needle positions will be used. I you put a 9mm foot, it has some sort of sensor that tells the machine to activate those additional needle positions. Since many feet--particularly special purpose feet like hemmers, are only available in 5.5mm versions, the 9mm machines ate designed to use both varieties with one exception-- the overlock foot 2/2A.

I only have one 9mm Bernina, the 820, and a few 5.5mm ones. When I put a 5.5mm foot, or a non-coded foot (foot without a prism), it defaults to 5.5mm max stitch width on my 820. Actually, a list of feet pops up onscreen, and if I choose a straight-stitch only foot from the list, it limits to a straight stitch.

If you were using a foot not on the list, or a generic foot with the adaptor, you could tell the machine that you've got either the ss or 5.5mm plate on (even if you don't) and that would also limit the stitch width.

I was pleasantly shocked at how well the #13 (ss) and #8 (jeans) feet work with the wider feed dogs on my 820, after not being too impressed with the #37 foot for piecing. I love it for the 5.5mm Berninas, but I currently use 34D or 1D on the 820 for most 1/4" seams.

If you put a 5.5mm foot on a 9mm machine, only the center-most needle positions will be used. I you put a 9mm foot, it has some sort of sensor that tells the machine to activate those additional needle positions. Since many feet--particularly special purpose feet like hemmers, are only available in 5.5mm versions, the 9mm machines ate designed to use both varieties with one exception-- the overlock foot 2/2A.

You can use more than a center needle ss depending on which machine is used... someone elaborated on the 8s already.
I have a 640 and I go back and forth regularly. I can set a safety manually but since this (and other) models don't automatically restrict your stitch width be sure to check if you're not using a straight stitch.

I go back and forth all the time. The 640 is a 9mm but I have a lot of other 5.5 mm feet (in addition to ones like the hemmer available only as a narrower foot in the example above) I use.
The best example of when/why is the general foot 1 or clear version 24. While I have both in the coded 9mm and non coded 5.5 mm, I often go to the narrower foot to get into narrower areas. If I'm doing something like a shirt collar band or small pieces I use the smaller foot even on the larger feed dogs.
I used to do this regularly with my 9mm Pfaff as well.

I now sew on mostly 9mm machines, but my favorite feet are the 5.5mm feet for most jobs. If it's available, I'll also switch to a 5.5mm throat plate, as the wide opening of the 9mm throat plate sometimes allows fabric to rock ever so slightly into the opening. (Currently the 5.5mm throat plate isn't yet available for my B 780. I anxiously await its availability.)
I also love the 13 foot and the 8 foot. Both narrow width feet for Berninas.

As if i didn't have enough feet!...BUT I just added non coded 5mm feet in the classic style because the feet are smaller in a lot of cases which makes working around corners and tight spaces a bit easier. If you look at a 1 vs. a 1C you will see a huge difference. Some of the feet are no different but some are very different.

------"The problem is not that there are problems. The problem is expecting otherwise and thinking that having problems is a problem." Theodore Rubin
"Life isn't about finding yourself. Life's about creating yourself." George Bernard Shaw
Dan 9:24-27

I have put 7-9mm on my 5mm machines because my TOLs have/had many more feet than my others. No problem yet. They may hang over the dogs but fabric goes through fine and needle positions can all be used.
Going the other way (narrower foot on wider feed dogs) I can see where you would have to be cautious.

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